This invention relates to the use of titanium in a supported chromium catalyst.
Supported chromium catalysts have long been a dominant factor in the production of high density olefin polymers such as polyethylene. As originally commercialized, these catalysts were used in solution polymerization processes. However, it early became evident that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. However, certain control techniques which are easily carried out in solution polymerization systems become more difficult in the slurry system. This is particularly true with regard to control of molecular weight. In a solution system, the temperature can simply be increased in order to provide a lower molecular weight, higher melt flow polymer. However, in slurry systems, there is a practical limit on the temperature increases, since the point is quickly reached wherein the polymer goes into solution and thus the value of the slurry system is lost.
In order to allow maximum utilization of the more commercially desirable slurry system, modifications of the catalyst system have been developed so as to allow the production of higher melt flow polymer. One such modification which has been utilized is to use a titanium compound in conjunction with the support. In many instances, it is desirable to use a water soluble organic titanium compound which is incorporated into the support at the hydrogel stage. However, the very few organic titanium compounds which are water soluble and hydrolysis resistant such as triethanolamine titanate tend to produce an odor in the resultant catalyst during drying. More desirable titanium compounds such as titanium isopropoxide are not water soluble or readily hydrolize to TiO.sub.2 which does not afford the desired promoting effect needed, e.g. they impart little or no melt index improvement to the polymers.